| 17 Sep 2026
Intra-tidal bed evolution on a macrotidal dissipative beach during a storm: contrasting roles of extremely shallow and relatively deep water stages
Abstract. Intra-tidal morphodynamics on macrotidal dissipative beaches remain poorly understood due to challenges in capturing continuous bed-level evolution under shallow-water conditions. Here, we used a high-resolution vertical probe (Argus Surface Meter, ASM) to obtain minute-scale, continuous records of bed elevation and near-bed suspended sediment concentration (SSC) in the intertidal zone across Extremely Shallow Water Stages (ESWS, water depths < 0.3 m) and Relatively Deep Water Stages (RDWS, water depths > 0.3 m) during storm attenuation. Our results reveal three key findings: (1) The relative dominance of ESWS and RDWS in bed-level evolution reverses as storm decays. RDWS drives the most significant changes under storm conditions, while the contribution of ESWS increases substantially as storm decays, becoming predominant in fair weather when bed shear stress falls below the critical threshold in RDWS. (2) A consistent flood-phase erosion–ebb-phase accretion pattern characterizes ESWS, whereas RDWS exhibits state-dependent behavior: under storms, this pattern is maintained by tide-modulated sediment supply—with active scouring during flood due to high transport capacity and deposition during ebb due to overwhelming sediment delivery from the migrating breaker zone. In fair weather, it reverses to flood-accretion/ebb-erosion due to settling of sediment during flood and subsequent resuspension during ebb. (3) the interplay between ESWS and RDWS highlights the critical role of complete ESWS process in sustaining dissipative beach stability. Based on these findings, we propose a conceptual model integrating ESWS and RDWS, providing a basis for predicting intertidal morphodynamic evolution in tide-dominated systems under changing storms.
This manuscript examines the hydromorphodynamic response of a macrotidal dissipative beach during Typhoon Doksuri. Based on high-resolution field measurements, it presents a valuable dataset on hydrodynamics, suspended sediment concentration, and bed-level change during very shallow-water stages under storm conditions. Continuous bed-level observations across the transition between inundation and exposure are still rare, especially during storms. The comparison between shallow- and deeper-water stages may therefore provide useful insight into morphodynamic processes on macrotidal beaches.
Overall, the manuscript is clearly organized and contains novel observations linking hydrodynamic forcing, suspended sediment dynamics, and bed response. The study is potentially suitable for publication, but several minor points require clarification or more cautious interpretation.
The threshold of (h = 0.3) m is used to separate RDWS and ESWS. At present, this threshold appears to be mainly determined by the minimum water depth required for the RBR to remain submerged, rather than by a demonstrated transition in hydrodynamic or sediment-transport processes. The authors should clarify its physical meaning. In addition, the whole RDWS should not necessarily be described as representing “deeper-water surf-zone processes,” because (h > 0.3) m covers a broad range of water depths and may include both surf-zone and shoaling-zone conditions.
The authors should explain why (Hs = 0.3) m was selected as the threshold between storm and fair-weather conditions.
The manuscript defines a tidal cycle as the interval between two consecutive low tides. However, the “Duration” in Table 2 is exactly equal to the sum of ESWS and RDWS. For example, the 660 min duration of T1 equals 120 min of ESWS plus 540 min of RDWS. This suggests that the reported duration may refer to the inundation period rather than a complete low-tide-to-low-tide cycle. The statement that ESWS accounts for only 9.4%–19.5% of the tidal cycle may therefore be misleading. The authors should clearly define the denominator used in this calculation.
Figure 5 indicates that SSC was measured at 0.3 m above the bed, whereas Figure 6 refers to 0.03 m above the bed. These values should be checked and made consistent. The authors should also clarify whether near-bed SSC was calculated relative to the changing bed level or taken from a sensor at a fixed elevation.
The comparison between sandy beaches and muddy tidal flats in Section 5.2 should be made more cautiously. Similar wind speeds do not necessarily imply similar near-bed hydrodynamic conditions. Wave conditions, bed shear stress, sediment cohesion, and grain size should also be considered.
Section 5.1 interprets the relationship between tau and SSC as evidence that transport capacity exceeded local sediment supply during flood tide, while sediment-rich water was transported to the station as the surf zone migrated seaward during ebb tide. The conceptual model also assumes offshore flow, an external sediment source, and changes in the balance between sediment load and transport capacity. These interpretations are plausible, but they need stronger support. Where possible, the authors should discuss them in relation to cross-shore sediment flux and the direction of suspended-sediment transport.